1. Field of the Invention
The present invention relates to an ultra-compact motor, and a light amount adjusting apparatus and a lens barrel each of which uses the motor.
2. Related Background Art
FIG. 20 shows a conventional compact cylindrical stepping motor. A stator coil 205 is concentrically wound around a bobbin 201. The bobbin 201 is clamped and fixed between two stator yokes 206 in the axial direction. On the stator yokes 206, stator teeth 206a and 206b are arranged on the inner surface of the bobbin 201 alternately in its circumferential direction. The stator yoke 206, integrally formed with the stator teeth 206a, 206b, is fixed to a case 203 to form a stator 202.
A flange 215 and bearing 208 are fixed to one of two cases 203, and the other bearing 208 is fixed to the other case 203. A rotor 209 is formed by a rotor magnet 211 fixed to a rotor shaft 210. The rotor magnet 211 and the stator yoke 206 of the stator 202 form a radial gap portion therebetween. The rotor shaft 210 is rotatably supported between the two bearings 208.
In the conventional compact stepping motor shown in FIG. 20, however, the cases 203, bobbins 201, stator coils 205, and stator yokes 206 are concentrically arranged around the rotor, and hence the outer dimensions of the motor become large. As shown in FIG. 21, the magnetic flux generated upon energization of the stator coil 205 mainly runs through end faces 206a1 and 206b1 of the stator teeth 206a and 206b. For this reason, the magnetic flux does not effectively act on the rotor magnet 211. Consequently, the output level of the motor does not rise remarkably.
The present applicant has proposed a motor that solves such a problem in U.S. Pat. No. 5,831,356. FIG. 22 shows this motor. In this motor, a rotor 311 formed by a magnet alternately magnetized to different poles at equal intervals in the circumferential direction is formed into a cylindrical shape. A first coil 312, the rotor 311, and a second coil 313 are sequentially arranged in the axial direction of the rotor. First outer magnetic poles 314a and 314b and first inner magnetic poles 314c and 314d, which are excited by the first coil 312, are opposed to the outer and inner circumferential surfaces of the rotor 311, respectively. Second outer magnetic poles 315a and 315b and second inner magnetic poles 315c and 315d, which are excited by the second coil 313, are opposed to the outer and inner circumferential surfaces of the rotor 311, respectively. A rotating shaft 317 serving as a rotor shaft is joined to the magnet of the cylindrical rotor 311.
A motor having such an arrangement can be reduced in outer dimensions, and the output level can be raised. In addition, if the magnet of the rotor 311 is formed thin, the distance between the first outer magnetic poles 314a and 314b and the first inner magnetic poles 314c and 314d and the distance between the second outer magnetic poles 315a and 315b and the second inner magnetic poles 315c and 315d can be reduced. That is, the reluctance of a magnetic circuit which acts on the magnet can be reduced. Therefore, a large amount of magnetic flux can be generated even by supplying small currents to the first and second coils 312 and 313.
A motor of the type disclosed in U.S. Pat. No. 5,831,356 is designed such that the magnet of the rotor 311 is held via an output shaft 317 and a bearing portion 314e (315e) of a stator 314 (315) that forms magnetic poles with certain gaps therebetween being ensured with respect to the outer magnetic poles 314a and 314b (315a and 315b) and inner magnetic poles 314c and 314d (315c and 315d) of the stator 314 (315). For this reason, when considering distortion of the output shaft, e.g., distortion due to a change in temperature, and the like, the gaps between the magnet and the outer and inner magnetic poles of the stator must be maintained with high precision. There is room for improvement in this point.
One aspect of this invention is to provide a compact motor including a cylindrical rotor formed by a magnet divided into equal portions in the circumferential direction, which are alternately magnetized with different polarities first and second coils positioned on two sides of the rotor in the axial direction, first and second outer magnetic pole portions which oppose the outer circumferential surfaces of the coils and magnet and excited by the first and second coils, respectively, and first and second inner magnetic pole portions which oppose the inner circumferential surfaces of the coils and magnet, wherein the gaps between the magnet and the respective magnetic pole portions can be defined with high precision using an annular member fitted on an inner circumferential surface of the magnet and regulating the position of the magnet in the radial direction.
In addition, this motor may include an annular member which is fitted on the outer circumferential surface of the magnet, instead of the inner circumferential surface, and which regulates the position of the magnet in the radial direction.
Furthermore, since the magnet rotatably fits within the annular member, a hollow compact motor can be realized.